1. The objective of this study was to compare, in freshwater bivalves, growth rates inferred from the spacing of internal annuli to those obtained directly by measuring annual changes in shell length. 2. The unionid mussels studied were Elliptio complanata and Lampsilis radiata from a pond in the north-eastern U.S.A. Age was inferred from internal lines of 157 E. complanata and twenty-five L. radiata. Actual length change was determined from comparisons of annual remeasurements of marked E. complanata (n = 520) in 1992-95 and L. radiata (n = 81) in 1993-95. 3. High retrieval rates of painted mussels demonstrated the efficacy of the marking method. 4. Annual length changes determined by remeasurement were significantly lower than annual length changes predicted by length-at-age data from internal annuli. Should this be a common occurrence, past estimates of annual growth based on annuli are probably too large, and unionid mussels may be much older than previously assumed.